Large-scale H i in nearby radio galaxies – II. The nature of classical low-power radio sources

Monthly Notices of the Royal Astronomical Society, Aug 2010

An important aspect of solving the long-standing question as to what triggers various types of active galactic nuclei (AGN) involves a thorough understanding of the overall properties and formation history of their host galaxies. This is the second in a series of papers that systematically study the large-scale properties of cold neutral hydrogen (H i) gas in nearby radio galaxies. The main goal is to investigate the importance of gas-rich galaxy mergers and interactions among radio-loud AGN. In this paper, we present results of a complete sample of classical low-power radio galaxies. We find that extended Fanaroff & Riley type-I radio sources are generally not associated with gas-rich galaxy mergers or ongoing violent interactions, but occur in early-type galaxies without large (>rsim 108 M⊙) amounts of extended neutral hydrogen gas. In contrast, enormous discs/rings of H i gas (with sizes up to 190 kpc and masses up to 2 × 1010 M⊙) are detected around the host galaxies of a significant fraction of the compact radio sources in our sample. This segregation in H i mass with radio-source size likely indicates that either these compact radio sources are confined by large amounts of gas in the central region or that their fuelling is inefficient and different from the fuelling process of classical FR I radio sources. To first order, the overall H i properties of our complete sample (detection rate, mass and morphology) appear similar to those of radio-quiet early-type galaxies. If confirmed by better statistics, this would imply that low-power radio-AGN activity may be a short and recurrent phase that occurs at some point during the lifetime of many early-type galaxies.

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Large-scale H i in nearby radio galaxies – II. The nature of classical low-power radio sources

B. H. C. Emonts 3 R. Morganti 0 1 C. Struve 0 1 T. A. Oosterloo 0 1 G. van Moorsel 7 C. N. Tadhunter 6 J. M. van der Hulst 0 E. Brogt 4 5 J. Holt 9 N. Mirabal 2 8 0 Kapteyn Astronomical Institute, University of Groningen , PO Box 800, 9700 AV Groningen , the Netherlands 1 Netherlands Institute for Radio Astronomy , Postbus 2, 7990 AA Dwingeloo , the Netherlands 2 Dpto. de F sica Atomica, Molecular y Nuclear, Universidad Complutense de Madrid , E-28040 Madrid , Spain 3 Australia Telescope National Facility, CSIRO Astronomy and Space Science , PO Box 76, Epping, NSW 1710 , Australia 4 University Centre for Teaching and Learning, University of Canterbury , Private Bag 4800, Christchurch 8140 , New Zealand 5 The University of Arizona , Steward Observatory, 933 North Cherry Avenue, Tucson, AZ 85721 , USA 6 Department of Physics and Astronomy, University of Sheffield , Sheffield S3 7RH 7 National Radio Astronomy Observatory , Socorro, NM 87801 , USA 8 Department of Astronomy, Columbia University , Mail Code 5246, 550 West 120th Street, New York, NY 10027 , USA 9 Leiden Observatory, Leiden University , PO Box 9513, 2300 RA Leiden , the Netherlands A B S T R A C T An important aspect of solving the long-standing question as to what triggers various types of active galactic nuclei (AGN) involves a thorough understanding of the overall properties and formation history of their host galaxies. This is the second in a series of papers that systematically study the large-scale properties of cold neutral hydrogen (H I) gas in nearby radio galaxies. The main goal is to investigate the importance of gas-rich galaxy mergers and interactions among radio-loud AGN. In this paper, we present results of a complete sample of classical low-power radio galaxies. We find that extended Fanaroff & Riley type-I radio sources are generally not associated with gas-rich galaxy mergers or ongoing violent interactions, but occur in early-type galaxies without large (10 8 M ) amounts of extended neutral hydrogen gas. In contrast, enormous discs/rings of H I gas (with sizes up to 190 kpc and masses up to 2 1010 M ) are detected around the host galaxies of a significant fraction of the compact radio sources in our sample. This segregation in H I mass with radio-source size likely indicates that either these compact radio sources are confined by large amounts of gas in the central region or that their fuelling is inefficient and different from the fuelling process of classical FR I radio sources. To first order, the overall H I properties of our complete sample (detection rate, mass and morphology) appear similar to those of radio-quiet early-type galaxies. If confirmed by better statistics, this would imply that low-power radio-AGN activity may be a short and recurrent phase that occurs at some point during the lifetime of many early-type galaxies. 1 I N T R O D U C T I O N Active galactic nuclei (AGN) are believed to be triggered when gas and matter are deposited on to a super-massive black hole in the centre of the host galaxy. For this to happen, the gas needs to lose sufficient angular momentum to be transported deep into the potential well of the galaxy, until it eventually fuels the AGN. Many different mechanisms have been proposed for transporting the gas down to the nuclear region, from galaxy mergers and interactions (e.g. Heckman et al. 1986; Lin, Pringle & Rees 1988; Colina & de Juan 1995; Wu et al. 1998; Canalizo & Stockton 2001; Kuo et al. 2008) to bars and central spiral structures (e.g. Schlosman, Frank & Begelman 1989; Prieto, Maciejewski & Reunanen 2005) to cooling flows and accretion of circumgalactic hot gas (e.g. Fabian & Rees 1995; Allen et al. 2006; Best et al. 2006). Undoubtedly, many of these fuelling mechanisms do occur; since various classes of AGN (e.g. quasars, Seyferts, radio galaxies, etc.) are found in different environments and are known to have intrinsic differences (other than simply orientation-dependent properties; see e.g. Urry & Padovani 1995), it is likely that certain mechanisms are associated with specific types of AGN (see e.g. Martini 2004, for a review). Nearby radio galaxies form a particularly interesting group of active galaxies for investigating possible AGN fuelling mechanisms. Their radio continuum sources evolve over time and both fuelling characteristics and dynamical interaction with their surrounding host galaxies can reflect in easily observable properties of these sources. This allows us to estimate the time-scale since the onset of the current episode of AGN activity as well as to match radio source characteristics with host galaxy properties and possible fuelling mechanisms. For example, compact radio sources [in particular, the gigahertz peaked spectrum (GPS) and compact steep spectrum (CSS) sources] are often believed to be young radio sources. Interactions between their radio jets (which can be imaged at high resolution with very long baseline interferometry observations) and t (...truncated)


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B. H. C. Emonts, R. Morganti, C. Struve, T. A. Oosterloo, G. van Moorsel, C. N. Tadhunter, J. M. van der Hulst, E. Brogt, J. Holt, N. Mirabal. Large-scale H i in nearby radio galaxies – II. The nature of classical low-power radio sources, Monthly Notices of the Royal Astronomical Society, 2010, pp. 987-1006, 406/2, DOI: 10.1111/j.1365-2966.2010.16706.x